Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a substrate like a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Both sides of a wafer can be inspected during semiconductor manufacturing. To inspect the backside of a wafer, the wafer is generally held such that it does not damage or contaminate the top surface of wafer where devices will be made. A non-contact chuck with edge grippers can be used to hold the wafer during backside inspection. The backside surface under inspection needs to be held flat under the optical system's field of view. The restrictions on the allowed slope of wafer surface can be stringent, such as being on the order of approximately 100 μrad.
Various forces contribute to undesirable slope of the wafer surface. These forces include deformation due to gravity, pre-existing wafer warp, and the forces caused by the edge grippers. Such forces can exacerbate the slope of a wafer that was already warped prior to chucking. Thus, the edge grippers can negatively affect inspection of the backside of the wafer by worsening wafer slope. Edge grippers can exacerbate these problems even more during wafer spinning. Furthermore, wafers of different warps cannot be flattened reliably. More complex warp shapes cannot be flattened using previous techniques. Therefore, what is needed are improved chucks for hold wafers.